IMDs are used to treat, monitor and assist in diagnosing patients suffering from a variety of conditions. Examples of IMDs include implantable pacemakers and ICDs, which are electronic medical devices that monitor the electrical activity of the heart and provide electrical stimulation to one or more of the heart chambers, when necessary. For example, a pacemaker may sense an arrhythmia and provide appropriate low-energy electrical stimulation pulses in a controlled manner in order to overdrive and thus, correct the arrhythmia, and restore heart rhythm. The types of arrhythmias that may be detected and corrected by pacemakers include bradycardias, which are unusually slow heart rates, and certain tachycardias, which are unusually fast heart rates. For such tachycardia episodes a pacemaker can employ so-called anti-tachycardia pacing (ATP) in an attempt to restore rhythm by essentially “peeling-back” the underlying rapid rhythm.
As is known, ICDs also detect arrhythmias and provide appropriate electrical stimulation pulses to selected chambers of the heart to correct the abnormal heart rate. In contrast to pacemakers, however, an ICD can also provide pulses that are much stronger and less frequent. This is because ICDs are generally designed to correct fibrillation, which is a rapid, unsynchronized quivering of one or more heart chambers, and severe tachycardias, where the heartbeats are very fast but coordinated. To correct such arrhythmias, an ICD delivers a low, moderate, or high-energy shock to the heart.
Pacemakers and ICDs are preferably designed with shapes that are easily accepted by the patient's body while minimizing patient discomfort. As a result, the corners and edges of the devices are typically designed with generous radii to present a package having smoothly contoured surfaces. It is also desirable to minimize the volume occupied by the devices as well as their mass to further limit patient discomfort.
Energy for IMDs typically includes batteries and capacitors. Batteries and capacitors are volumetrically constrained systems. The size or volume of components that go into a battery (cathode, anode, separator, current collectors, electrolyte, etc.) cannot exceed the available volume of the battery case. The arrangement of the components affects the amount or density of active electrode material contained within the battery case.
Conventional lithium batteries can also employ an electrode configuration sometimes referred to as the “jelly roll” design, in which the anode, cathode, and separator elements are overlaid and coiled up in a spiral wound form. A strip sheet of lithium or lithium alloy comprises the anode, a cathode material supported on a charge collecting metal screen comprises the cathode, and a sheet of non-woven material often separates the anode and cathode elements. These elements are combined and wound to form a spiral. Typically, the battery configuration for such a wound electrode would be cylindrical. An advantage of this design is that there need not be anode material that is not mated to cathode material. Such designs therefore have the potential for an improved match between the cathode and anode components and improved uniformity of anode and cathode utilization during discharge. However, cylindrical cells would not achieve the same space utilization inside the case of an implantable defibrillator as would a prismatic cell shape.
Batteries used in IMDs currently use a chemistry that incorporates a lithium anode to obtain high energy density for the IMDs. For a given volume, higher energy density can translate into a larger amp hour capacity, which can mean a longer useful life for the IMD, or greater functionality. In one example, for a given battery capacity, a higher energy density typically enables a smaller battery configuration, and thus perhaps a smaller overall IMD.
Another prime consideration for batteries in IMDs is safety, as is known if a battery experiences an electrical short a large amount of heat can be generated. In addition, an external electrical short (e.g., an electrical path between IMD components external to the interior of the cell) can cause high current flow and a higher than desirable battery temperature. Of course, an electrical short located within a battery (e.g., between cathode and anode), can cause localized locations of intense heat due to very high current flow. Common commercially available batteries using a lithium or lithium-ion chemistry often have a single porous separator sheet to separate the cathode from the anode, to prevent unwanted direct contact between anode and cathode. In the event of such direct contact, it is possible for the battery overheating to cause the separator to melt. If the porous separator melts in place, to close the pores and form a perfect, impermeable film, then the ion flow between the cathode and anode will be reduced, and the overheating reduced as well. If, however, the porous separator melts in such a way as to further open the pores and to pull back under tension and/or coalesce to form relatively large holes, then temperature self-regulation may not occur. Recently some IMD manufacturers have used two separators between the cathode and anode. This approach provides an improved, somewhat fault-tolerant IMD battery assembly.
In order to better guard against such separator failure caused by overheating, a tri-layer separator has been developed, often having a microporous polyethylene layer laminated between two microporous polypropylene layers. The polyethylene is selected to melt at a lower temperature than the polypropylene, so that the pores in the polyethylene close while being held in place by the adjacent sheets. The tri-layer separators are of high quality. However, there has been concern that a tri-layer separator might be imperfect, might have an undetected hole, and might not totally block ion flow at high temperature. To alleviate any such concerns, some IMD manufacturers used two tri-layer separators between the cathode and anode. This arrangement provides a potentially very safe IMD battery assembly.
Using double tri-layer separators provides safety, but increases battery electrical resistance. The increased resistance is due to ions flow through two separator sheets rather than one. Increased resistance also increases the time required to charge capacitors coupled to the battery.
What would be desirable is an implantable battery that includes a thermal shutdown separator that provides the safety of present thermal shutdown separators, but with less resistance to current flow. It would also be desirable that batteries have safe, but thinner, thermal shutdown separators.